1. Field of the Invention
The present invention relates to electroluminescent display matrix screens formed of a set of light-emitting diodes. These are for example screens formed of organic diodes (“OLED”, for Organic Light Emitting Display) or polymer diodes (“PLED” for Polymer Light Emitting Display). The present invention more specifically relates to the regulation of the precharge voltage of the control circuits of the light-emitting diodes of such screens.
2. Discussion of the Related Art
FIG. 1 shows an example of a matrix screen 10 with light-emitting diodes. Each pixel of screen 10 is formed of a light-emitting diode 12. Diodes 12 are arranged in Y lines and X columns. The cathodes of diodes 12 of a same line are connected to a line electrode 14, and the anodes of diodes 12 of a same column are connected to a column electrode 16.
The display of an image on screen 10, according to currently-used standards, is obtained by the display of a frame or of two successive frames. On display of a frame, the addressing of matrix screen 10 is performed line after line via a circuit for controlling lines 18 (commonly called a line driver). The electrode of line 14 of the selected or active line is connected to ground while the line electrodes of the inactive lines are left at high impedance or are connected to a high voltage. Simultaneously, the information corresponding to the activation or to the non-activation of diodes 12 of the active line will be transmitted by column electrodes 16 via a circuit for controlling columns 20 (commonly called a column driver) which injects a current into column electrodes 16 connected to diodes 12 to be activated.
FIG. 2 shows a more specific modeling of a pixel of matrix screen 10 of FIG. 1. Each pixel is formed of a non-resistive and non-capacitive light-emitting diode 12 in parallel with a stray capacitor 22. For a 300-μm2 pixel formed of an organic or polymer light-emitting diode, such a stray capacitor may have a capacitance on the order of 25 picofarads. A first resistor 24 in series with diode 12 represents the resistance of the portion of column electrode 16 connected to the pixel. A second resistor 25 in series with diode 12 represents the resistance of the portion of line electrode 14 connected to the pixel.
Due to the very capacitive character of the pixels, part of the current in the activation of a pixel will first be necessary to charge stray capacitor 22 to the voltage at which diode 12 must operate. A portion only of the current is thus used for the light emission. The luminance of diode 12 will be proportional to the average time during which diode 12 carries a current and to the average value of this current. As an example, the power consumption of an activated pixel of a matrix display with organic light-emitting diodes can be broken out into a power consumption for the light emission of diode 12 of the pixel, which amounts to approximately 57% of the total power consumption, a parasitic power consumption, of approximately 40%, linked to the capacitive character of the pixel, and a resistive power consumption, of approximately 3%, linked to series resistors 24, 25 of the pixel.
The time required to charge the stray capacitance 22 associated with the pixel defines the turn-on duration of the pixel and reduces the duration of the active phase corresponding to the light emission of the pixel. The turn-on duration especially depends on the intensity of the current provided to the pixel to be activated. The global duration of a pixel addressing phase being constant, the longer the turn-on duration, the lower the achieved luminance will be for a same current flowing through diode 12.
To solve such a disadvantage, a precharge of all the pixels of a matrix display 10 can be performed before selection of a screen line. The addressing with precharge enables biasing each pixel of screen 10 to a voltage close to that that it would have if it was active so that the current injected into a diode 12 to be activated is only used for the light emission and not for charging stray capacitance 22 of the pixel.
FIGS. 3A to 3C describe successive steps of an addressing with precharge of the pixels.
In FIGS. 3A to 3C, a single column electrode 16 of screen 10 of FIG. 1 has been shown and a single pixel 26, connected to column electrode 16, which is desired to be activated, has been isolated. Pixel 26 is represented by a diode 12 and an associated stray capacitance 22 (parasitic resistors 24, 25 are not shown). Line electrode 14 connected to pixel 26 has been shown and the other line electrodes of screen 10 have been symbolized by a single branch 14′ connected to the anode of diode 12. A capacitor 22′ is shown on branch 14′ and is equivalent to the assembly of the stray capacitors in parallel of the pixels connected to column electrode 16 and to the other line electrodes of screen 10. The capacitance of capacitor 22′ is substantially equal to (Y−1) times the capacitance of a stray capacitor 22.
Only the specific elements of the column control circuit 20 associated with the considered column electrode 16 have been shown, knowing that such elements are identical for each column electrode of screen 10.
Line control circuit 18 comprises two switches 27, 28 enabling connecting line electrode 14 alternately to ground GND or to a high voltage VOFF. Only line electrode 14 being activated, for the other screen lines, the line control circuit has been symbolized by two switches 27′, 28′ enabling connection of branch 14′ alternately to ground GND or to high voltage VOFF.
Column control circuit 20 comprises three switches 31, 32, 33 enabling connection of column electrode 16 alternately to ground GND, to a precharge voltage VPRE, or to a first terminal of a current source ILUM. The second terminal of current source ILUM is connected to a bias voltage source VPOL.
FIG. 3A shows a first step of an addressing with precharge consisting, between the successive selection of two lines of screen 10, of discharging all the pixels of screen 10. All the screen lines are then inactive, which means that all line electrodes 14, 14′ of screen 10 are connected to high voltage VOFF. Each column electrode 16 is then connected to ground GND, via switch 31, to discharge stray capacitors 22, 22′ of all the pixels connected to column electrode 16.
FIG. 3B shows a second step consisting, before selection of a line, of charging all the pixels of screen 10. All line electrodes 14, 14′ remain connected to high voltage VOFF. Each column electrode 16 is brought to a precharge voltage VPRE via switch 32. Stray capacitor 22 of each pixel is then precharged to voltage VPRE-VOFF. Precharge voltage VPRE is close to the voltage at which column electrode 16 may operate on activation of pixels at the next step.
FIG. 3C show a third step, or active phase, corresponding to the activation of pixel 26. Line electrode 14 connected to pixel 26 to be activated is connected to ground GND via switch 27. Line electrodes 14′ of the inactive lines remain connected to high voltage VOFF. Current source ILUM is connected to pixel 26 via switch 33. A current can thus flow through diode 12 which emits light. Current source ILUM only has to charge capacitor 22 having a capacitance which is (Y−1) times as small as the capacitance of capacitor 22′, which very slightly affects the turn-on time of diode 12. The voltage on the anode of diode 12 settles at an operating voltage VCOL.
The first discharge step aims at discharging the stray capacitors 22 of all the screen pixels to erase the residual charges of the pixels which might result from the activation of pixels of screen 10 at previous steps.
The second precharge step enables reducing the turn-on duration of the pixel to obtain an active phase duration which is substantially independent from the intensity of the lighting, that is, from the intensity of the current flowing through the diodes in active phase.
It is also possible to only perform a precharge of the screen columns to be activated, as described in U.S. Pat. No. 5,594,468.
The light-emitting diodes of a screen are not identical and, for a same luminance current, the voltage across activated diodes may be different. However, since such differences are generally relatively small, the same precharge voltage is applied to each selected column to simplify the column control circuit.
Conventionally, the precharge voltage is predefined, for example, empirically, and remains constant during the screen operation. However, a predefined precharge voltage is generally not optimal. Indeed, the operating voltage of a selected column may significantly vary according to luminance current ILUM that can change for each selected line. Further, for a same luminance current flowing through a light-emitting diode, the voltage across the diode tends to increase along with the diode aging. For a same luminance, corresponding to a given luminance current, the operating voltage of the column thus varies along time.
Upon selection of a column, the voltage applied onto the selected column switches from the precharge voltage to the operating voltage. The precharge voltage can thus not be too distant from the operating voltage of the column to avoid modifying the luminosity of the activated light-emitting diode. Indeed, if the precharge voltage is too high, too high a current must temporarily be conducted by the activated light-emitting diode, the active line then appearing with a light intensity greater than the desired light intensity. Conversely, if the precharge voltage is too small, the voltage of each selected column must rise from the precharge voltage up to the operating voltage. The current flowing through the active light-emitting diode may be temporarily smaller than the desired value, the active line then appearing with a light intensity smaller than the desired light intensity.